Projection displays have numerous home and commercial applications. For example, rear-projection television displays commonly occupy numerous households. Displays have also been important as information-conveying devices, such as those used in aircraft to provide terrain or flight information data. For example, avionics displays are used to provide a generally visually reliable indication of in-bound terrain, aircraft status, or other flight information that may be used for real-time decision-making.
Lenses are prominently used in projection displays for imaging. In an ideal situation, a point on an object or subject is imaged to a point on a displayed image. However, individual lenses in general may have inherent defects that result in chromatic aberrations such as blur or distortion. To minimize chromatic aberrations, lens systems are used where individual lenses are selected and combined to offset or counter chromatic aberrations for each of the individual lenses. Selecting the particular combination of lenses to minimize chromatic aberrations remains a complex and difficult process, and chromatic aberrations commonly remain in the projected image although minimized to a degree.
When designing a wide angle, fast lens for use with a broad spectral distribution, chromatic aberrations of such lens can be particularly difficult to correct. For example, compact projection displays used in bright ambient conditions commonly have relatively smaller display size and a larger field of view. To operate in high ambient conditions, the lens used in such displays may need to be fast as well. The limited types of lamps used in such displays may further complicate matters. For example, Mercury lamps commonly used in avionics displays are difficult to correct for about 435 nm light. The use of special glass types having undesirable properties may be required to correct chromatic aberrations.
An additional problem that arises from the use of certain types of light sources is a mismatch of the projection display white point with a desired white point. When this happens, it is generally preferable to alter the light source spectrum to obtain an appropriate color. For light sources having independently adjustable color or wavelength bands, such as a cathode ray tube, altering the light source spectrum to obtain the appropriate color is relatively straightforward. Certain types of light sources may have a fixed spectral distribution. For example, in such light sources the relative amount of light in different color bands can be difficult to independently adjust in the source. Changing the spectral distribution of these light sources may be possible, but generally this change is difficult after manufacturing. Typically, any change to these light sources may be done by changing the total light output or, alternatively, the range of achievable change is relatively small. Arc lamps and single die white light emitting diodes (LEDs) are examples of these types of sources. Often, one or more of the color bands used in a projection display system will have more light than needed to obtain the desired white point. If this is the case, one or more of the color bands generally should be adjusted to remove extra light and move the white point to the desired location. This adjustment can be done with light valves that are used to create an image, but it is more desirable to maintain the full performance range of the light valve for image generation.
Accordingly, it is desirable to provide projection display systems having minimized chromatic aberrations. It is also desirable to correct the white point of a display that uses a source with a fixed spectral distribution. In addition, it is desirable to provide lens systems for use in projection displays having chromatic vignetting to correct chromatic aberrations. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.